The aggressive cost reduction of radar and communication solutions can only be realistically achieved by the highest level of integration. For highest integration density, the use of a digitally controlled oscillator in an all digital phased-locked loop is favored since it circumvents analog tuning voltages and filters. At microwave frequencies fundamental local oscillator signal generation becomes a challenge as with increasing frequency the limited Q-factor of the digitally controlled oscillator LC-tank impairs the phase noise. Therefore, local oscillator signal generation by combination of all digital phased-locked loop with frequency multiplier is an attractive choice regarding overall phase noise and integration density. For proper operation of downconversion receivers, the frequency multiplier should deliver sufficient output power to saturate the mixer at his local oscillator port. An unbalanced or pseudo balanced local oscillator signal generation with output power of around 0 dBm increases the risk of local oscillator leakage [1] compared to balanced local oscillator signal generation which can cause a serious 1/f noise impairment [2] in direct conversion receivers. Following this consideration, a balanced frequency multiplier with output power higher than 0 dBm is favored for implementation. One frequency doubler concept is a common-source circuit with matched second harmonic at the output [3]-[6]. However, these doublers are unbalanced and show only fair fundamental rejection and output power lower than 0 dBm with low efficiency. Another way of frequency doubling is the usage of a Gilbert cell fed by two signals of equal frequency at the local oscillator and radio frequency port. This approach suffers from a DC offset at the output and an imbalance due to local oscillator feedthrough. As demonstrated in [7], it is not possible to achieve truly balanced signaling meaning amplitude and phase balance simultaneously in a single Gilbert cell. In [8] the unwanted imbalance due to local oscillator feedthrough is compensated by a second Gilbert cell which is excited with a phase difference of 90°. Furthermore, in a Gilbert cell, more active devices have to be excited by the input signal which lowers the efficiency. Doublers which appear most in literature are push-push doublers. Push-push doublers have been successfully demonstrated at various frequencies, technologies, and with high output power [9]-[13]. A push-push doubler is inherently balanced at the input and unbalanced at the output. If a balanced input and output is intended, one needs a transformer balun at the output forming a balanced output out of the unbalanced node. In this sense, the doubler is pseudo balanced since it incorporates an unbalanced node. The transformer balun introduces undesired losses. In order to overcome losses introduced by a transformer balun and to avoid local oscillator leakage in receivers, a truly balanced doubler is desired.
Balanced frequency doublers that are truly balanced cannot be found in the literature. The term “truly balanced frequency doubler” hereinafter refers to doublers without any unbalanced node within the circuit except nodes for DC supply and biasing.